Question

Which compound is added to petrol to prevent engines from 'knocking'?

• A. Carbon
• B. Lead
• C. Both A and B
• D. None of these

Answer 1

Answer: (B)

Lead

• In modern car engines, the petrol vapor-air mixture is highly compressed before it is sparked, in order to get the maximum energy from the burning fuel. However, some hydrocarbons tend to ignite under pressure before they are sparked so that the engine runs roughly; this is known as "knocking".
  
• In 1922, an American called Thomas Midgely (who also invented CFCs) found that if tetraethyl lead was put into petrol, particles of lead and lead oxide PbO are formed on combustion. This helps the petrol to burn more slowly and smoothly, preventing knocking.
  
• Hence Lead compound is added to petrol to prevent engines from 'knocking'.
• So,the correct answer is 'Lead'.